1. Technical Field
This invention relates to cross-correlation circuits and, more particularly, to techniques for determining correlation between an analog signal and a preselected code.
2. Discussion
Analog-digital cross-correlator networks are used in a variety of applications where it becomes necessary to determine whether a received analog signal contains a preselected pattern. For example, pulse compression techniques used in radar systems impress a digital code onto the signal which is transmitted. The radar receiver includes a cross-correlator for determining when the echo contains this digital code. The echo is in the form of an analog signal whose voltage typically has positive and negative polarities determined by the digital ones and zeros of the impressed code. Among the tasks of the cross-correlator is to determine whether the echo contains this predetermined pattern. If so, useful information such as the range, speed, etc. of the target can be determined.
One known cross-correlator is implemented by digitizing the received analog signal and performing the well known cross-correlation sum digitally. While this technique can be carried out in a relatively straight forward manner, this all digital approach can be costly because high speed digitizers are difficult to build, as well as usually being physically large and expensive. Another prior implementation makes use of a charged coupled device (CCD). In this application, the CCD is used initially as a serial analog memory and then as a tapped analog shift register to perform the cross-correlation sum. The pattern against which the analog samples are correlated is determined by the length of the CCD output tapped electrodes. This implementation likewise suffers from several disadvantages. The accuracy of this approach is degraded because the samples are read in serially causing some information to be lost during each shift through the CCD stages. A high sample input rate can also result in stringent requirements for the CCD clock driver circuits. The clock drivers then can be difficult to produce and, hence, costly. In addition, CCD implementations tend to have large DC offsets which have to be compensated for by additional circuitry. One major disadvantage is that the cross-correlation code is hard wired into the CCD and therefore it is not programmable. This can cause problems when the code must be kept secret, such as in military applications. Another disadvantage is that the manufacturing process for CCD's generally requires specialized fabrication techniques which can result in availability and cost problems.
Other implementations that would make use of more easily fabricated charge transfer devices (e.g., bucket brigades) normally do not operate at sufficiently high input sample rates that are required in many applications such as the pulse-compression radar application noted above.
Still other implementations utilize surface acoustic wave (SAW) devices. The SAW device implementation, as with the CCD approach, has the correlation code built in resulting in nonprogrammability. Costs and availability are also a problem with SAW devices. Generally, the SAW device must operate with a fixed data rate established by the velocity of the surface acoustic waves. In a radar pulse-compression application, SAW devices operate at high IF frequencies rather than at video levels and tend to have insertion losses which must be compensated for by additional circuitry.